1. Field of the Invention
The present invention relates generally to the field of automated storage and retrieval systems, in particular to systems which store and retrieve objects with a high transaction frequency.
2. Description of the Related Art
Automated storage systems have been used to increase productivity by reducing the number of workers needed to store and retrieve objects. Automated systems offer improved accuracy and reliability over manual systems, which are subject to human errors.
The speed of storage and retrieval operations is often critical to the success of an automated system. This is particularly the case when the system includes a large number of separate storage racks serviced by multiple mechanical transports. The use of minimal personnel in the system makes it desirable to include a rapid automated system for transferring objects between the shipping/receiving area and any of the storage racks.
Several previous attempts have been made to increase the speed of delivery between the shipping/receiving area and the ultimate slot in which an object is stored. U.S. Pat. No. 3,738,506 to Cornford shows multiple transports simultaneously storing and retrieving objects. Cornford includes two transports, one for storage and one for retrieval. Each transport moves an elevator to the row in which the desired slot is located. A self-propelled satellite transport is released from the elevator to move along the row to the desired slot to store/retrieve the object.
The method for selecting storage locations at the time of storage is just as important as the speed of the mechanical transports used in order to enhance the speed of subsequent retrieval operations. In previous systems, storage locations were either selected based on predetermined assignments, or the objects were placed in the closest location to the receiving station. Neither of these methods provides the fastest total storage or retrieval time when multiple transports are available to perform storage and retrieval operations, and shipping orders require more than one of the same item.
Another concern with automated systems is inventory turnover. If every item is stored in the nearest storage location to the receiving station, and objects are also retrieved from the nearest location, then inventory will turn over in a last-in, first-out fashion. If a first-in, first-out method is desired to limit the age of the oldest items in stock, then an increase in retrieval time will be experienced.
U.S. Pat. No. 5,002,449 to Kita discloses multiple transports which are simultaneously and independently dispatched to fetch or return objects. Inner pickers (transports) each move within a single row (only horizontally) and transfer the objects to/from an outer picker which travels vertically between the inner pickers and a conveyor. Since the inner pickers are each assigned to a single horizontal row, collisions are avoided. Objects are either stored at fixed addresses, or are stored in a location which minimizes the travel distance of the inner picker during storage.
Another area of concern for warehousing systems is the use of a method for facilitating collection and ensuring the integrity of inventory data. U.S. Pat. No. 5,006,996 to Nakamura shows a storage system using forklifts in which the address of the slot is checked prior to storing or retrieving an object. Nakamura includes the process of checking the slot before storage and performing an inventory. When an inventory of the stored objects is desired, the identification from each stored object is read and transmitted to a data processing unit for comparison with the previously recorded inventory data.